Basic Algorithm; Features Of Multi-Ended Line Differential; Algorithm Overview - GE P4A Technical Manual

Chapter 6 - Current Differential Protection
2.3.3

BASIC ALGORITHM

The multi-ended differential sample based algorithm employs the RMS value, in which the instantaneous
differential current is the sum of instantaneous current of all terminals:
M
i ( ) n i ( )
∑
=
diff Tm
m 1
=
Where, i is the current of mth terminal; M is the number of terminals; i
Tm
is present sample number.
The RMS value of the differential current is used for the discriminative criteria:
1
I ( ) n
=
diff
N
k n N
= − +
Where, I is the RMS value of i
diff
cycle of fundamental frequency; n is present sample; k is history sample number within the window length N.
Similarly, the I , which is the sum of the RMS value of currents of all terminals:
bias
1
I ( ) n
=
Tm
N
k n N
= − +
M
1
I ( ) n [
∑
=
bias
2
m 1
=
Where, I is RMS value of bias current; I
bias
length in samples for RMS calculation, which is selected as a cycle of fundamental frequency; n is present sample
number; k is the history sample number within the window length N.
2.3.4

FEATURES OF MULTI-ENDED LINE DIFFERENTIAL

To practically implement a Multi-ended differential system, the following technical challenges must be resolved:
Charging current: The capacitive current is caused by the equivalent shunt capacitance of lines. For short line/
cable, the capacitive current is negligible, multi-ended line differential should work without problem. But for line
more than 50km or cable more than 10km, higher pickup setting could be used to prevent fault relay operation,
but this will desensitise the protection. The capacitive current is recommended to be compensated to achieve
sensitive protection. Therefore, with VT input, a new capacitive current compensation algorithm for distributed
parameter line is developed to achieve high sensitivity in multi-ended differential.
Data synchronization: The remote data should be aligned with the local measured data. Due to the
communication time delay from remote end to local end as well as due to the un-synchronized sampling between
the relays at local end and remote ends data is not synchronised. Ping-Pong is employed for data synchronization.
CT Saturation: When the fault is severe, the CTs at different terminals could be unbalanced and saturated, which
results in high unbalanced current in differential current when the external fault occurs. The multi-ended line
differential must include an enhance CT saturation technique to detect internal / external faults and indirectly
reduce the CT requirement.
The above features will be discussed in detail in the following section.
2.3.5

ALGORITHM OVERVIEW

The overall protection scheme is constructed by incorporating many small functions which enables functionality of
multi-end differential to be as accurate as possible.
102
n
n
2
i ( ) k
∑
diff
1
; N is the window length in samples for RMS calculation, which is selected as a
diff
n
2
i ( ) k
∑
Tm
1
I ( )] n
Tm
is RMS value of i
Tm
is instantaneous differential current; n
diff
; M is the number of terminals; N is the window
Tm
P54A/B/C/E
P54xMED-TM-EN-1